Maybe people think they are stupid because they are so human orientated and eager to please? Border Collies are also extremely eager to please and are often considered the smartest breed.... but I most certainly do not consider Taggart to be smart, lol. He's eager to please, sure. And very, very willing to learn. But he does next to zero thinking on his own and has to be guided all the way through.
Now, that might just be my silly Border collie (and he is a mix after all...) but I have also noticed the same over eagerness in other border collies, so I imagine they must be somewhat similar. *shrug*
Anyway, I've met dozens of labradors and had several at my training classes. They usually are the best students, since they naturally tend to be good at focusing on their owners (that and they are usually also very food oriented and thus glue themselves to anyone offering hotdogs! ).
That lack of independence however, is probably what makes some people think of them as stupid.